Method for initiating and maintaining a substoichiometric operating mode of an internal combustion engine and internal combustion engine for carrying out a method of this kind

ABSTRACT

The present disclosure describes systems and methods for recharging a storage catalyst of an internal combustion engine. A method is described, comprising: while operating an engine in a substoichiometric operating mode when the engine is under medium load and responsive to an LNT condition, assisting the engine with an electric machine connected to an engine crankshaft. The electric machine provides an auxiliary drive to assist the engine in maintaining the substantially steady state substoichiometric operating mode which may be used to reduce NO x  or SO x  build up in a storage catalytic convertor or to assay the condition of a storage catalytic convertor.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to German Patent Application No.102012202679.7 filed on Feb. 22, 2012, the entire contents of which arehereby incorporated by reference for all purposes.

TECHNICAL FIELD

The present application relates to exhaust gas discharge for internalcombustion engines.

BACKGROUND AND SUMMARY

Normally and in the context of the present disclosure, the air ratio λis defined as the ratio of the air mass m_(Air,actual) actually fed tothe at least one cylinder of the internal combustion engine to thestoichiometric air mass m_(Air,stoic) that would be just enough tooxidize completely the fuel mass m_(Fuel) fed to the at least onecylinder (stoichiometric operation of the internal combustion engineλ=1). The following applies: λ=m_(Air,actual)/m_(Air,stoic) and, withthe stoichiometric air requirement L_(stoic), which is defined byL_(stoic)=m_(Air,stoic)/m_(Fuel), the air ratio is given byλ=m_(Air,actual)/m_(Fuel)*(1/L_(stoic)).

Internal combustion engines are fitted with various exhaust gasaftertreatment systems to reduce pollutant emissions. In the case ofspark ignition engines, catalytic reactors are employed, using catalyticmaterials which increase the rate of certain reactions to ensureoxidation of HC and CO, even at low temperatures. If nitrogen oxidesNO_(x) are additionally to be reduced, this can be achieved by the useof a three-way catalyst, although this utilizes stoichiometric operation(λ≈1) of the spark ignition engine within narrow limits. In this case,the nitrogen oxides NO_(x) are reduced by means of the availablenon-oxidized exhaust gas components, namely the carbon monoxides CO andthe unburned hydrocarbons HC, and the exhaust gas components aresimultaneously oxidized.

In the case of internal combustion engines which are operated with anexcess of air, e.g. lean burn spark ignition engines and directinjection diesel engines, as well as direct injection spark ignitionengines, the nitrogen oxides NO_(x) in the exhaust gas cannot be reducedowing to the absence of reducing agents.

As a result, an exhaust gas aftertreatment system may be provided inorder to reduce the nitrogen oxides, e.g. a storage catalyst, which isalso referred to as an LNT (Lean NOx Trap). In this case, the nitrogenoxides are absorbed, e.g. collected and stored, in the catalyst during alean-mixture operating mode (λ>1) of the internal combustion engine, andare then reduced during a regeneration phase with a substoichiometricoperating mode (λ<1) of the internal combustion engine with a deficiencyof oxygen, wherein the unburned hydrocarbons HC and the carbon monoxideCO in the exhaust gas serve as reducing agents. Further possibilitieswithin the engine for enriching the exhaust gas with reducing agent, inparticular with unburned hydrocarbons, are offered by exhaust gasrecirculation (EGR) and, in the case of diesel engines, throttling inthe intake section. Enrichment of the exhaust gas with unburnedhydrocarbons can also be achieved by an afterinjection of fuel into atleast one cylinder of the internal combustion engine. One disadvantageof the latter mentioned procedure is, in particular, dilution of theoil. It is possible to introduce hydrocarbons directly into the exhaustsection, e.g. by injection of additional fuel upstream of the LNT,thereby dispensing with injection into the cylinder itself.

During the regeneration phase, the nitrogen oxides (NO_(x)) are releasedand are substantially converted into nitrogen dioxide (N₂), carbondioxide (CO₂) and water (H₂O). The temperature of the storage catalystmay preferably be within a temperature window between 200° C. and 450°C., on the one hand ensuring rapid reduction and, on the other hand,preventing desorption without conversion of the nitrogen oxides NO thatare released again, something that can be triggered by excessivetemperatures.

One difficulty in using a storage catalyst results from the sulfurcontained in the exhaust gas, which is likewise absorbed and which hasto be removed at regular intervals in a process referred to asdesulfurization. For this purpose, the storage catalyst may be heated tohigh temperatures, generally between 600° C. and 700° C., and suppliedwith a reducing agent, e.g. unburned hydrocarbons. The high temperaturesutilized for desulfurization may damage the storage catalyst, contributeto thermal aging of the catalyst and significantly reduce the desiredconversion of nitrogen oxides toward the end of its life.

The storage capacity or ability to store nitrogen oxides decreases asthe time in operation of the LNT increases, this being attributable tothe contamination of the storage catalyst with sulfur, e.g. to theaccumulation of sulfur, and also to thermal aging due to the hightemperatures.

In addition to regeneration, e.g. cleaning of the LNT, which may becarried out at regular intervals, and desulfurization, the low nitrogenoxide emission limits specified by law may in future require onboarddiagnosis (OBD) in order to monitor or detect the limitation in abilityto function, e.g. the decrease in conversion, that can be expected asthe time in operation of the LNT increases.

The technical relationships described above describe the advantages ofmethods for substoichiometric operation of an internal combustion enginein order to clean and desulfurize an LNT. On the other hand, however,also methods may be advantageous for monitoring the ability to functionof the LNT to ensure that undesirably high pollutant emissions due to alimited ability to function or lack of conversion are reliably avoided.

Transient operating conditions make it considerably more difficult tomaintain a constant air ratio and, in isolated cases, may even make itimpossible since it is not possible to follow the input by the drivervia the gas pedal without a delay, and especially because the operatingparameters that determine the air ratio, namely the air mass and fuelquantity, can be adjusted and adapted to the new operating conditionswith a delay and at different speeds.

In the range of relatively high, high and maximum loads (see FIG.2—range 202), initiation and maintenance of a substoichiometricoperating mode is generally governed by the maximum permissible exhaustgas temperature, with the exhaust gas temperature often being limited bycomponents provided in the exhaust gas discharge system or by thethermal load bearing capacity of said components, e.g. by the turbine ofan exhaust gas turbocharger, an exhaust gas aftertreatment system or theexhaust gas recirculation system. In this context, it may be taken intoaccount that the exhaust gas temperature generally increases when themixture is enriched.

As regards the methods for monitoring or checking the ability tofunction of a storage catalyst, it may be stated that these methodslikewise often utilize a substoichiometric operating mode of theinternal combustion engine. Here, maintaining a constant orsubstantially constant air ratio λ is of decisive importance.

European Patent Application EP 1 936 140 A1 describes a method formonitoring a storage catalyst using two lambda probes, or oxygensensors, in which a measuring error of the lambda probes is exploited.More specifically, if the unburned hydrocarbons in the exhaust gasexceed a certain concentration, the probe outputs a higher value for theair ratio λ_(meas) than is actually present, e.g. an air ratio ofλ_(meas)=0.95 as a measured variable in the case of a substoichiometricoperating mode (λ<1) of the internal combustion engine and an HCconcentration of 10,000 ppm in the exhaust gas, even though the airratio is actually λ_(actual)=0.85.

To check the ability to function of the storage catalyst, the HCconcentration in the exhaust gas is deliberately increased in such a waythat the first probe, which is arranged upstream of the storagecatalyst, operates incorrectly. If the storage catalyst is not capableof functioning properly, e.g. the storage capacity is at least limited,no more unburned hydrocarbons or less unburned hydrocarbons are oxidizedby the release of nitrogen oxide NO_(x) and the HC concentrationdownstream of the storage catalyst is essentially exactly the same asupstream of the storage catalyst, for which reason both lambda probesoutput the same value—subject to a measurement error of equalmagnitude—for the air ratio. The storage catalyst is therefore assumedto be incapable of functioning properly if the air ratios λ_(1,meas),λ_(2,meas) determined by means of probes are of substantially equalmagnitude and λ_(1,meas)/λ_(2,meas)≈1.

If, on the other hand, the storage catalyst is still capable offunctioning properly, the unburned hydrocarbons in the exhaust gas areat least partially oxidized in the storage catalyst as they flowthrough, for which reason the HC concentration in the exhaust gasdownstream of the storage catalyst will be lower than upstream of thecatalyst. Thus, the storage catalyst will be assumed to be at leastpartially capable of functioning if the two air ratios λ_(1,meas),λ_(2,meas) are of different magnitudes with λ_(1,meas)/λ_(2,meas)>1.Here, the air ratio λ_(2,meas) determined with the second probe, whichis arranged downstream of the storage catalyst, does not necessarilyhave to be free from measurement error. However, the deviation of theair ratio λ_(2,meas) from the actual air ratio λ is at least less thanupstream of the storage catalyst.

The method described in EP 1 936 140 A1 is dependent on the maintenanceof a constant or substantially constant air ratio λ and requiressteady-state operation of the internal combustion engine.

The German patent application with the file reference 102012200006.2likewise describes a method for monitoring a storage catalyst in whichrespective lambda probes for detecting the air ratio λ are arrangedupstream and downstream of the storage catalyst. To check the ability tofunction of the storage catalyst, the internal combustion engine isswitched to a substoichiometric operating mode (λ<1) for a specifiableperiod of time, in which mode however—in contrast to the methoddescribed in EP 1 936 140 A1—both probes operate without error. Themethod can also be carried out in the non-steady-state operating mode ofthe internal combustion engine but also requires the maintenance of aconstant or substantially constant air ratio λ.

The inventors recognize the aforementioned disadvantages and hereindisclose systems and methods for initiating and maintaining aspecifiable substoichiometric (λ<1) operating mode of an internalcombustion engine in accordance with the preamble of claim 1.

The present disclosure describes systems and methods for recharging astorage catalyst of an internal combustion engine. A method, comprising:while operating an engine in a substoichiometric operating mode when theengine is under medium load and responsive to an LNT condition,assisting the engine with an electric machine connected to an enginecrankshaft. The electric machine provides an auxiliary drive to assistthe engine in maintaining the substantially steady statesubstoichiometric operating mode which may be used to reduce NO_(x) orSO_(x) build up in a storage catalytic convertor or to assay thecondition of a storage catalytic convertor.

In the present disclosure, the exhaust gas is enriched with unburnedhydrocarbons as a reducing agent by means of a substoichiometricoperating mode (λ<1) of the internal combustion engine. However, furthermeasures for enrichment may be provided.

After initiation of enrichment, the substoichiometric operating mode ofthe internal combustion engine, once established, with a substantiallyconstant air ratio λ=constant is maintained by satisfying additionalpower demand by means of an electric machine, which can be connected tothe drive train of the internal combustion engine. The electric machinemay serve as selectable auxiliary drive when operating in thesteady-state substoichiometric mode.

An internal combustion engine with the assistance of the electricmachine may continue to operate in a steady-state mode. This ensuresthat the air ratio λ does not vary due to a change in operatingparameters of the internal combustion engine. Transient operatingconditions, under which the air mass and the fuel quantity have to beadapted to changed boundary or operating conditions, may be avoided.

The above advantages and other advantages, and features of the presentdescription will be readily apparent from the following DetailedDescription when taken alone or in connection with the accompanyingdrawings.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure. Further, the inventors herein have recognized thedisadvantages noted herein, and do not admit them as known.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an internal combustion engine.

FIG. 2 diagrams the load T against the engine speed n of a firstembodiment of the internal combustion engine in the form of a diagram.

FIG. 3 is a high level flowchart depicting a method in accordance withthe present disclosure.

DETAILED DESCRIPTION

The present disclosure describes a system and method to operate aninternal combustion engine in a substantially steady-statesubstoichiometeric operating mode. An electric machine may be connectedto a crankshaft of the internal combustion engine to serve as anauxiliary drive, assisting the engine in maintaining the substantiallysteady substoichiometric operating mode which may be utilized toregenerate exhaust gas aftertreatment systems, such as storagecatalysts.

FIG. 1 depicts an example embodiment of a combustion chamber or cylinderof internal combustion engine 10. Engine 10 may receive controlparameters from a control system including controller 12 and input froma vehicle operator 130 via an input device 132. In this example, inputdevice 132 includes an accelerator pedal and a pedal position sensor 134for generating a proportional pedal position signal PP. Cylinder (hereinalso “combustion chamber”) 14 of engine 10 may include combustionchamber walls 136 with piston 138 positioned therein. Piston 138 may becoupled to crankshaft 140 so that reciprocating motion of the piston istranslated into rotational motion of the crankshaft. Crankshaft 140 maybe coupled to at least one drive wheel of the passenger vehicle via atransmission system. Further, a starter motor may be coupled tocrankshaft 140 via a flywheel to enable a starting operation of engine10.

In the present disclosure crank shaft 140 may be further connected to anelectric machine 141 which may assist in engine operation at certainloads under rich operating conditions. Embodiments of the method inwhich the electric machine which may be connected to the crank shaft tothe internal combustion engine is used to absorb excess power suppliedby the internal combustion engine as a selectable, generator, thusmaintaining the steady-state operating mode of the internal combustionengine, are advantageous. The electric machine may be powered by arechargeable power source, the power source recharged by the excessenergy produced by the internal combustion engine. Embodiments of theinternal combustion engine in which an additional battery is providedfor the electric machine are advantageous. Embodiments of the internalcombustion engine in which a capacitor is provided for the electricmachine are also advantageous.

The electric machine may not function as an additional drive in that theelectric machine may not function as a selectable auxiliary drive whichfeeds power into the drive train in addition to the internal combustionengine. Instead the electric machine may function as a generator, whichabsorbs the mechanical energy or power supplied in excess by theinternal combustion engine, e.g. the excess power, and stores the energyin a battery or capacitor. The capacitor stores the energy in the formof separate electric charges and is distinguished by the possibility ofa rapid discharge, e.g. the ability to supply large quantities of energyquickly.

Cylinder 14 can receive intake air via a series of intake air passages142, 144, and 146. Intake air passage 146 may communicate with othercylinders of engine 10 in addition to cylinder 14. In some embodiments,one or more of the intake passages may include a boosting device such asa turbocharger or a supercharger. For example, FIG. 1 shows engine 10configured with a turbocharger including a compressor 174 arrangedbetween intake passages 142 and 144, and an exhaust turbine 176 arrangedalong exhaust passage 148. Compressor 174 may be at least partiallypowered by exhaust turbine 176 via a shaft 180 where the boosting deviceis configured as a turbocharger. However, in other examples, such aswhere engine 10 is provided with a supercharger, exhaust turbine 176 maybe optionally omitted, where compressor 174 may be powered by mechanicalinput from a motor or the engine. A throttle 20 including a throttleplate 164 may be provided along an intake passage of the engine forvarying the flow rate and/or pressure of intake air provided to theengine cylinders. For example, throttle 20 may be disposed downstream ofcompressor 174 as shown in FIG. 1, or alternatively may be providedupstream of compressor 174.

If the internal combustion engine is a boosted engine, additionalaccount may be taken of the boost pressure on the intake side, which canchange with the load and/or engine speed and affects the fresh airquantity and hence the exhaust gas quantity thus affecting a method ofthe present disclosure as discussed below herein.

In the case of an internal combustion engine which is not pressurecharged, the fresh air quantity and the exhaust gas quantity correspondsapproximately to the speed and/or load of the internal combustionengine, specifically in accordance with the type of load controlemployed in the individual case. In the case of a traditional sparkignition engine with quantity control, the exhaust gas quantityincreases with increasing load, even when the engine speed is constant,whereas, in the case of traditional diesel engines with quality control,the exhaust gas quantity is exclusively dependent on the engine speedbecause, when there is a load change and a constant engine speed, themixture composition but not the mixture quantity varies.

Exhaust passage 148 may receive exhaust gases from other cylinders ofengine 10 in addition to cylinder 14. Exhaust gas sensor 128 is showncoupled to exhaust passage 148 upstream of emission control device 178.Sensor 128 may be selected from among various suitable sensors forproviding an indication of exhaust gas air/fuel ratio such as a linearoxygen sensor or UEGO (universal or wide-range exhaust gas oxygen), atwo-state oxygen sensor or EGO (as depicted), a HEGO (heated EGO), aNOx, HC, or CO sensor, for example. Emission control device 178 may be athree way catalyst (TWC), NOx trap, various other emission controldevices, or combinations thereof.

The exhaust gas flow volume decreases with the exhaust gas temperatureor with the load, irrespective of the type of load control used. Thus, astorage catalyst situated in the exhaust gas discharge system can bemade smaller, e.g. dimensioned with a smaller volume, if, in accordancewith the method according to the disclosure, the internal combustionengine is operated in the intermediate load range and an electricmachine which may be connected to the crank shaft of the internalcombustion engine is used as a selectable auxiliary drive. This mayreduce costs.

The lower exhaust gas volume flows allow a storage catalyst of smallerdimensions without a reduction in the space velocity relevant toconversion. At the same time, an arrangement of the storage catalystcloser to the engine can be chosen without the exhaust gas backpressureassuming or exceeding impermissible values. This latter option hasadvantages especially as regards heating up of the LNT.

Exhaust temperature may be measured by one or more temperature sensors(not shown) located in exhaust passage 148. Alternatively, exhausttemperature may be inferred based on engine operating conditions such asspeed, load, air-fuel ratio (AFR), spark retard, etc. Further, exhausttemperature may be computed by one or more exhaust gas sensors 128. Itmay be appreciated that the exhaust gas temperature may alternatively beestimated by any combination of temperature estimation methods listedherein.

Each cylinder of engine 10 may include one or more intake valves and oneor more exhaust valves. For example, cylinder 14 is shown including atleast one intake poppet valve 150 and at least one exhaust poppet valve156 located at an upper region of cylinder 14. In some embodiments, eachcylinder of engine 10, including cylinder 14, may include at least twointake poppet valves and at least two exhaust poppet valves located atan upper region of the cylinder.

Intake valve 150 may be controlled by controller 12 by cam actuation viacam actuation system 151. Similarly, exhaust valve 156 may be controlledby controller 12 via cam actuation system 153. Cam actuation systems 151and 153 may each include one or more cams and may utilize one or more ofcam profile switching (CPS), variable cam timing (VCT), variable valvetiming (VVT) and/or variable valve lift (VVL) systems that may beoperated by controller 12 to vary valve operation. The operation ofintake valve 150 and exhaust valve 156 may be determined by valveposition sensors (not shown) and/or camshaft position sensors 155 and157, respectively. In alternative embodiments, the intake and/or exhaustvalve may be controlled by electric valve actuation. For example,cylinder 14 may alternatively include an intake valve controlled viaelectric valve actuation and an exhaust valve controlled via camactuation including CPS and/or VCT systems. In still other embodiments,the intake and exhaust valves may be controlled by a common valveactuator or actuation system, or a variable valve timing actuator oractuation system. A cam timing may be adjusted (by advancing orretarding the VCT system) to adjust an engine dilution in coordinationwith an EGR flow thereby reducing EGR transients and improving engineperformance.

Cylinder 14 can have a compression ratio, which is the ratio of volumeswhen piston 138 is at bottom center to top center. Conventionally, thecompression ratio is in the range of 9:1 to 10:1. However, in someexamples where different fuels are used, the compression ratio may beincreased. This may happen, for example, when higher octane fuels orfuels with higher latent enthalpy of vaporization are used. Thecompression ratio may also be increased if direct injection is used dueto its effect on engine knock.

In some embodiments, each cylinder of engine 10 may include a spark plug192 for initiating combustion. Ignition system 190 can provide anignition spark to combustion chamber 14 via spark plug 192 in responseto spark advance signal SA from controller 12, under select operatingmodes. However, in some embodiments, spark plug 192 may be omitted, suchas where engine 10 may initiate combustion by auto-ignition or byinjection of fuel as may be the case with some diesel engines.

In the case of a spark ignition engine with direct injection as shown inFIG. 1, the air ratio λ may be adjusted by means of the quantity of fuelinjected. For adjustment of the quantity of air supplied, and hence ofthe load, a throttle valve 20 is provided in the intake system. Thethrottle valve is, likewise, subject to open-loop or closed-loop controlby the engine control system 12.

As a non-limiting example, cylinder 14 is shown including one fuelinjector 166. Fuel injector 166 is shown coupled directly to cylinder 14for injecting fuel directly therein in proportion to the pulse width ofsignal FPW received from controller 12 via electronic driver 168. Inthis manner, fuel injector 166 provides what is known as directinjection (hereafter also referred to as “DI”) of fuel into combustioncylinder 14. While FIG. 1 shows injector 166 as a side injector, it mayalso be located overhead of the piston, such as near the position ofspark plug 192. Fuel may be delivered to fuel injector 166 from a highpressure fuel system 8 including fuel tanks, fuel pumps, and a fuelrail. Alternatively, fuel may be delivered by a single stage fuel pumpat lower pressure, in which case the timing of the direct fuel injectionmay be more limited during the compression stroke than if a highpressure fuel system is used. Further, while not shown, the fuel tanksmay have a pressure transducer providing a signal to controller 12. Itwill be appreciated that, in an alternate embodiment, injector 166 maybe a port injector providing fuel into the intake port upstream ofcylinder 14.

As described above, FIG. 1 shows one cylinder of a multi-cylinderengine. As such each cylinder may similarly include its own set ofintake/exhaust valves, fuel injector(s), spark plug, etc.

While not shown, it will be appreciated that engine may further includeone or more exhaust gas recirculation passages for diverting at least aportion of exhaust gas from the engine exhaust to the engine intake. Assuch, by recirculating some exhaust gas, an engine dilution may beaffected which may increase engine performance by reducing engine knock,peak cylinder combustion temperatures and pressures, throttling losses,and NOx emissions. The one or more EGR passages may include an LP-EGRpassage coupled between the engine intake upstream of the turbochargercompressor and the engine exhaust downstream of the turbine, andconfigured to provide low pressure (LP) EGR. The one or more EGRpassages may further include an HP-EGR passage coupled between theengine intake downstream of the compressor and the engine exhaustupstream of the turbine, and configured to provide high pressure (HP)EGR. In one example, an HP-EGR flow may be provided under conditionssuch as the absence of boost provided by the turbocharger, while anLP-EGR flow may be provided during conditions such as in the presence ofturbocharger boost and/or when an exhaust gas temperature is above athreshold. The LP-EGR flow through the LP-EGR passage may be adjustedvia an LP-EGR valve while the HP-EGR flow through the HP-EGR passage maybe adjusted via an HP-EGR valve (not shown).

Controller 12 is shown in FIG. 1 as a microcomputer, includingmicroprocessor unit 106, input/output ports 108, an electronic storagemedium for executable programs and calibration values shown as read-onlymemory chip 110 in this particular example, random access memory 112,keep alive memory 114, and a data bus. Controller 12 may receive varioussignals from sensors coupled to engine 10, in addition to those signalspreviously discussed, including measurement of inducted mass air flow(MAF) from mass air flow sensor 122; engine coolant temperature (ECT)from temperature sensor 116 coupled to cooling sleeve 118; a profileignition pickup signal (PIP) from Hall effect sensor 120 (or other type)coupled to crankshaft 140; throttle position (TP) from a throttleposition sensor; and manifold absolute pressure signal (MAP) from sensor124. Engine speed signal, RPM, may be generated by controller 12 fromsignal PIP. Manifold pressure signal MAP from a manifold pressure sensormay be used to provide an indication of vacuum, or pressure, in theintake manifold. Still other sensors may include fuel level sensors andfuel composition sensors coupled to the fuel tank(s) of the fuel system.

Storage medium read-only memory 110 can be programmed with computerreadable data representing instructions executable by processor 106 forperforming the methods described below as well as other variants thatare anticipated but not specifically listed.

FIG. 2 shows the load T plotted against the engine speed n of a firstembodiment of the internal combustion engine in the form of a diagram,e.g. what is referred to as an engine map. The full load characteristicand the various load ranges are plotted in FIG. 2.

In accordance with the disclosure, the substoichiometric operating modeof the internal combustion engine (λ_(constant)<1) is implementedselectively in the intermediate load range, in which the initiation andmaintenance of a reproducible stable combustion process may beunproblematic. Another significant advantage of rich-mixture operationin the intermediate load range results from the exhaust gastemperatures, which decrease from the range of relatively high loads torelatively low loads. The low exhaust gas temperatures in theintermediate load range enable the substoichiometric operating mode tobe maintained for longer, thus allowing prolonged cleaning ordesulfurization of the LNT, and allowing sufficient time to carry outchecking of functionality.

The intermediate load range 204, in which stable combustion may beproduced, even when the internal combustion engine is being operated ona rich mixture, is nevertheless characterized by transient operatingconditions.

The relatively low load range 206, in which initiating and maintainingrich-mixture operation with stable combustion may problems.

The range of relatively high, high and maximum loads 202, in which thesubstoichiometric operating mode may be restricted by the maximumpermissible exhaust gas temperature.

In a non-limiting example however a substoichiometric operating mode maybe enacted under a high low or medium load condition. In this example,the substoichiometric operating mode may occur without the assistance ofthe electric machine.

The engine map illustrated in FIG. 2 is intended to give a basicillustration of the three load ranges, medium load range is relative tothe maximal load range of a particular engine in proportions describedherein below.

In the relatively low load range operating mode may be initiated andmaintained with difficulty since stable combustion cannot be ensured(see FIG. 2 range 206). This may give rise to problems especially in thecase of diesel engines, the operating method of which is based oncompression ignition. There may be ignition failures or incompletecombustion of the mixture. The result may be undesirably high pollutantemissions, especially those of unburned hydrocarbons HC.

In the intermediate load range, the stability of combustion may posefewer or no problems (see FIG. 2 range 204). However, the intermediateload range may generally be an operating range of the internalcombustion engine in which there are frequent load changes due toacceleration or deceleration of the vehicle while driving, and hence isa non-steady-state operating range. Here, the change in load resultsfrom an additional power demand by the driver or excess power which issupplied by the internal combustion engine. Ascents or descents, achange in the rolling resistance or a change in the strength and/ordirection of the relative wind may also cause such a change in load.

In the intermediate load range, the exhaust gas quantities which ariseare often less than in the high and maximum load ranges. The associatedexhaust gas volume flows are generally lower since the exhaust gastemperatures decrease at the transition from high loads to intermediateloads and, with the temperatures, the volume flows likewise decreaseowing to the increased density.

If the internal combustion engine according to the disclosure is basedon quantity control, then, as the load falls, the exhaust gas quantitylikewise decreases. If the internal combustion engine is operated in theintermediate load range, the exhaust gas quantities are smaller than inthe case of relatively high loads.

If, on the other hand, the internal combustion engine is based onquality control, in which the load is controlled by way of thecomposition of the mixture and the exhaust gas quantity changes almostexclusively with the engine speed, the exhaust gas quantity does notchange with the load at a constant engine speed. This notwithstanding,switching the internal combustion engine to the intermediate load rangemay lead to a smaller exhaust gas quantity if the speed of the internalcombustion engine decreases at the same time.

In the range of relatively high, high and maximum loads (see FIG.2—range 202), initiation and maintenance of a substoichiometricoperating mode is generally governed by the maximum permissible exhaustgas temperature, with the exhaust gas temperature often being limited bycomponents provided in the exhaust gas discharge system or by thethermal load bearing capacity of the components, e.g. by the turbine ofan exhaust gas turbocharger, an exhaust gas aftertreatment system or theexhaust gas recirculation system. In this context, it may be taken intoaccount that the exhaust gas temperature may increase when the mixtureis enriched.

Referring now to FIG. 3, a flowchart depicting a method 300 inaccordance with the present disclosure is shown. The method may bestored in read-only memory 110 and enacted by engine controller 12.

The method 300 starts with an engine key on event. The method thenproceeds to 302 where it is assessed if engine load is in the mediumload range. If NO at 302, the method proceeds to 304 where a standardoperating air ratio λ is maintained until the engine load is in themedium load range. If at 302, the engine load is within the medium loadrange (YES) the method then proceeds to assessing various conditions ofthe storage catalytic convertor.

At 306, it is determined if NO_(x) levels in the LNT storage catalyticconvertor are above threshold levels. If YES at 306, the method thenproceeds to 308 where the engine is switched to a substoichiometricoperating mode for a period of time, Δt_(rich). If at 306, NO_(x) levelsare not below threshold (NO) the method then proceeds to 310.

At 310, it is determined if SO_(x) levels in the LNT storage catalyticconvertor are above threshold levels. If YES at 310 the method thenproceeds to 308 where the engine is switched to a substoichiometricoperating mode for a period of time, Δt_(rich). If at 310, SO_(x) levelsare not below threshold (NO) the method then proceeds to 312.

At 312, it is determined if it is time for a diagnostic check of the LNTstorage catalytic convertor. If YES at 312, the method then proceeds to308 where the engine is switched to a substoichiometric operating modefor a period of time, Δt_(rich). If at 312, it is not the timing for a adiagnostic check of the LNT storage catalytic convertor the methodproceeds to 314 where standard air ratio λ is maintained.

The timing for a diagnostic test may be based on operating conditions,such as AFR and engine speed in addition to being performed at mediumengine loads. Alternatively, the diagnostic test may be performed atregular intervals of distance covered, time elapsed, or on timing sincelast substoichiometric operating mode.

While the engine is operating in a substoichiometric operating mode theelectrical machine connected to the crank shaft is assisting the engineusing power stored in a battery or capacitor. The assistance by theelectric machine may be proportional to a degree of richness of theair-fuel ratio in a substoichiometric operating mode. In one example,the electric machine may assist the engine to a higher degree under morerich operation and to a lesser degree when the substoichiometricoperating mode uses a less rich mixture. In another example, theassistance of the electric machine may be inversely proportional suchthat the electric machine assists to a lesser degree as the engine isoperated at a higher degree of richness and the electric machinesassists to a higher degree when the engine is operated at a less richair-fuel ratio.

At 308 after Δt_(rich) has elapsed the method then proceeds to 314 wherethe engine reverts to standard air ratio λ operating mode. Once thesubstoichiometric operating mode is ended, assistance by the electricmachine may also end. The method then returns.

The period of time Δt_(rich) over which the substoichiometric operatingmode of the internal combustion engine is maintained depends on themeasure to be carried out. For example, whether an LNT arranged in theexhaust gas discharge system is to be cleaned, desulfurized or checkedfor its ability to function.

As regards the methods for monitoring or checking the ability tofunction of a storage catalyst maintaining a constant or substantiallyconstant air ratio λ is advantageous.

In accordance with the method according to the disclosure, the internalcombustion engine is switched to a substantially steady-state operatingmode, e.g. a substoichiometric operating mode of the internal combustionengine with a substantially constant air ratio λ. The word substantiallyis used with reference to the steady state operating mode as minorvariances in air ratio λ may be expected.

Embodiments of the method in which the internal combustion engine isswitched to a substoichiometric steady-state operating mode whereλ_(constant)<1 in the intermediate load range are advantageous.

A slight enrichment may be preferable to heavy enrichment since theintroduction of excess fuel may be disadvantageous both in terms ofenergy considerations, especially as regards the fuel consumption of theinternal combustion engine, and in view of pollutant emissions.

Embodiments of the method in which the internal combustion engine isswitched to a substoichiometric operating mode where 1>λ_(constant)>0.9are advantageous.

Embodiments of the method in which the internal combustion engine isswitched to a substoichiometric operating mode where 1>λ_(constant)>0.95are also advantageous.

Embodiments of the method in which the internal combustion engine isswitched to a substoichiometric operating mode where0.97>λ_(constant)>0.93 are especially advantageous.

The air ratio λ for the substoichiometric operation of the internalcombustion engine varies to maintain engine components. On the one hand,the enrichment performed may be as small as possible since thedisadvantages of enrichment are positively correlated with increasingenrichment. On the other hand, the exhaust gas may be enriched to suchan extent with incompletely oxidized combustion products, in particularunburned hydrocarbons, that the set goal is achieved.

In an embodiment where a storage catalyst is arranged in the exhaust gasdischarge system, the air ratio λ is partially determined by whether theLNT is to be cleaned, desulfurized or checked for its ability tofunction.

Embodiments of the method in which the internal combustion engine isswitched to a substoichiometric operating mode for a period of timeΔt_(rich)<45 seconds can be advantageous.

Embodiments of the method in which the internal combustion engine isswitched to a substoichiometric operating mode for a period of timeΔt_(rich)<15 seconds can also be advantageous.

Embodiments of the method in which the internal combustion engine isswitched to a substoichiometric operating mode for a period of timeΔt_(rich)<2 seconds, in order to check the ability to function of thestorage catalyst for example, are especially advantageous.

For the definition of the duration of enrichment, namely the period oftime Δt_(rich), what has already been stated above in connection withthe degree of enrichment applies in a similar manner. On the one hand,enrichment may be as short as possible so that the disadvantages ofenrichment are minimized. On the other hand, enrichment may last or becarried out for long enough to achieve the set goal.

Embodiments of the method in which the intermediate load range in whichthe internal combustion engine is switched to the substantiallysteady-state operating mode encompasses loads T_(mid) between 20% and80% of the maximum load T_(max,n) at the prevailing engine speed n areadvantageous.

Embodiments of the method in which the intermediate load range in whichthe internal combustion engine is switched to the substantiallysteady-state operating mode encompasses loads T_(mid) between 30% and70% of the maximum load T_(max,n) at the prevailing engine speed n areespecially advantageous.

Embodiments of the method in which the intermediate load range in whichthe internal combustion engine is switched to the substantiallysteady-state operating mode encompasses loads T_(mid) between 40% and60% of the maximum load T_(max,n) at the prevailing engine speed n arealso advantageous.

In the case of internal combustion engines which have a storage catalystas an exhaust gas aftertreatment system in the exhaust gas dischargesystem, embodiments of the method in which the internal combustionengine is switched to the substoichiometric operating mode(λ_(constant)<1) in order to regenerate the storage catalyst areadvantageous.

In the case of internal combustion engines which have a storage catalystas an exhaust gas aftertreatment system in the exhaust gas dischargesystem, embodiments of the method in which the internal combustionengine is switched to the substoichiometric operating mode(λ_(constant)<1) in order to desulfurize the storage catalyst are alsoadvantageous.

In the case of internal combustion engines which have a storage catalystas an exhaust gas aftertreatment system in the exhaust gas dischargesystem, embodiments of the method in which the internal combustionengine is switched to the substoichiometric operating mode(λ_(constant)<1) in order to check the lack of ability to function ofthe storage catalyst are also advantageous.

In checking the lack of ability to function of the LNT, use is generallymade of the fact that little or none of the unburned hydrocarbons in theexhaust gas are oxidized by the release of nitrogen oxides NO as theyflow through a storage catalyst with a limited ability to function, forwhich reason the HC concentration in the exhaust gas changes a smallextent, if at all. The release of nitrogen oxides in the storagecatalyst and the associated oxidation processes change an air ratio λdetermined metrologically downstream of the LNT.

Embodiments of the method in which the internal combustion engine isswitched to a specifiable operating point in the intermediate load rangebefore the internal combustion engine is switched to thesubstoichiometric operating mode (λ<1) are advantageous.

Since not all operating points in the intermediate load range areequally well suited to the substoichiometric operating mode of theinternal combustion engine, it is expedient and advantageous to selector provide a limited number of operating points in the intermediate loadrange for carrying out the method in order to avoid the storage ofparameters for a rich-mixture operating mode in the engine controlsystem for all points in the intermediate load range on thecharacteristic map.

Variants of the method in which the specified operating point isselected from a limited list of k operating points are advantageous inthis context.

Embodiments of the method in which the air ratio λ is reduced byincreasing the quantity of fuel injected in order to switch the internalcombustion engine to the substoichiometric operating mode areadvantageous.

In principle, the air ratio λ could also be reduced by reducing the airmass supplied. However, the disadvantage of such a procedure is that, byvirtue of the principle involved, a loss of power is associated with thereduction in the air mass. It is therefore preferable to reduce the airratio λ by increasing the quantity of fuel injected in accordance withthe embodiment under discussion.

The present disclosure describes systems and methods for recharging astorage catalyst of an internal combustion engine. A method isdisclosed, comprising: while operating an engine in a substoichiometricoperating mode when the engine is under medium load and responsive to anLNT condition, assisting the engine with an electric machine connectedto an engine crankshaft. The electric machine provides an auxiliarydrive to assist the engine in maintaining the substantially steady statesubstoichiometric operating mode. This operating mode may be used toreduce NO_(x) or SO_(x) build up in a storage catalytic convertor or toassay the condition of a storage catalytic convertor. Use of asubstoichiometric operating mode of the present disclosure may reduceemissions.

It will be appreciated that the configurations and methods disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types. The subject matter ofthe present disclosure includes all novel and non-obvious combinationsand sub-combinations of the various systems and configurations, andother features, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

1. A method, comprising: initiating and maintaining a substoichiometricoperating mode of an internal combustion engine with a substantiallyconstant air ratio, wherein, the internal combustion engine has anexhaust gas discharge system for discharging exhaust gases and at leastone exhaust gas aftertreatment system arranged in the exhaust gasdischarge system; and switching the internal combustion engine to asubstoichiometric operating mode for a specifiable period of time,wherein, the internal combustion engine is switched to asubstoichiometric steady-state operating mode in an intermediate loadrange; and connecting an electric machine to a crankshaft of theinternal combustion engine to be used as a selectable auxiliary drive,when operating the internal combustion engine in the substoichiometricsteady-state operating mode.
 2. The method as claimed in claim 1,wherein the internal combustion engine is switched to thesubstoichiometric steady-state operating mode where λ<1 in theintermediate load range.
 3. The method as claimed in claim 1, whereinthe internal combustion engine is switched to the substoichiometricsteady-state operating mode where 1>λ>0.9.
 4. The method as claimed inclaim 1, wherein the internal combustion engine is switched to thesubstoichiometric steady-state operating mode where 1>λ>0.95.
 5. Themethod as claimed in claim 1, wherein the internal combustion engine isswitched to the substoichiometric steady-state operating mode where0.97>λ>0.93.
 6. The method as claimed in claim 1, wherein theintermediate load range in which the internal combustion engine isswitched to the substoichiometric steady-state operating mode comprisesloads between 20% and 80% of a maximum load at a prevailing enginespeed.
 7. The method as claimed in claim 1, wherein the intermediateload range in which the internal combustion engine is switched to thesteady-state operating mode comprises loads between 30% and 70% of themaximum load at the prevailing engine speed.
 8. The method as claimed inclaim 1, wherein the intermediate load range in which the internalcombustion engine is switched to the steady-state operating modecomprises loads between 40% and 60% of the maximum load at theprevailing engine speed.
 9. The method as claimed in claim 1, whereinthe electric machine absorbs excess power supplied by the internalcombustion engine as a selectable generator.
 10. The method as claimedin claim 1, further comprising switching the internal combustion engineto a specifiable operating point in the intermediate load range beforethe internal combustion engine is switched to the substoichiometricsteady-state operating mode.
 11. The method as claimed in claim 10,wherein the specifiable operating point is selected from a limited listof operating points.
 12. The method as claimed in claim 1, whereininitiating a substoichiometric steady-state operating mode is achievedby reducing λ by increasing a quantity of fuel injected.
 13. An internalcombustion engine, comprising: an exhaust gas discharge system; at leastone exhaust gas aftertreatment system arranged in the exhaust gasdischarge system; an electric machine connected to a crankshaft of theinternal combustion engine, the electric machine serving as a selectableauxiliary drive for power output and as a generator for powerabsorption; and a storage catalyst in the exhaust gas discharge system.14. A method, comprising: while operating an engine in asubstoichiometric operating mode when the engine is under medium loadand responsive to an LNT condition, assisting the engine with anelectric machine connected to an engine crankshaft.
 15. The method ofclaim 14, wherein the condition of the LNT storage convertor is a NO_(x)level.
 16. The method of claim 14, wherein the condition of the LNTstorage convertor is a SO_(x) level.
 17. The method of claim 14, whereinthe condition of the LNT storage convertor is a duration since lastdiagnostic test.
 18. The method of claim 14, wherein the electricmachine is powered by a rechargeable power storage unit.
 19. The methodof claim 14, further comprising adjusting an amount of the assistingbased on engine load and a degree of richness in the substoichiometricoperating mode.
 20. The method of claim 14, further comprising duringhigh and low load conditions, operating the engine in thesubstoichiometric operating mode responsive to the LNT condition withoutassisting the engine with the electric machine.